Electrical release circuit for a passenger protection system

ABSTRACT

The invention relates to a passenger protection system which is provided with a safing switch ( 20 ). If only one ignition stage ( 2 ) of a passenger protection airbag which can be ignited in two stages is ignited by a control device ( 6 ), the safing switch being closed, a circuit module ( 16 ) output ( 46 ) that is parallel to the safing switch is switched to a signal mode corresponding to the closed safing switch. In this way, the second ignition stage ( 4 ) can then be ignited by the control device ( 6 ) even when the safing switch is no longer closed.

[0001] Method for releasing the firing capability of at least two firing stages which can be fired separately in a vehicle occupant protection system and electric release circuit for a vehicle occupant protection system.

[0002] The invention relates to a method for releasing the firing capability of at least two firing stages which can be fired separately in a vehicle occupant protection system. The invention also relates to an electric release circuit for a vehicle occupant protection system having at least two firing stages according to the preamble of the main claim. The invention can be applied in particular for motor vehicle occupant protection systems.

[0003] In motor vehicle occupant protection systems, airbags are being increasingly used which have two gas generators which are each equipped with their own firing stage. In this way, the inflation of the airbag can be adapted better to the respective requirements. If the second inflation stage is not required to protect the persons to be protected when there is an accident or if its firing even constitutes a hazard, it is possible for the second stage not to be fired directly after the first stage. However, so that helpers or rescue service personnel are not put in danger as a result of the second stage firing, it is expedient to fire the second stage automatically after the first stage, but at a time at which the person to be protected is no longer placed in danger. For it to be at all possible to fire a firing stage, a safing switch must be closed. Such a safing switch generally closes even when there are decelerations below a threshold value at which it is necessary to inflate the airbag.

[0004] If, as in the case described above, the firing of the second firing stage takes place with a long delay with respect to that of the first firing stage, the safing switch is opened again so that it is no longer possible to fire the second stage.

[0005] The invention is based on the object of providing a remedy for the abovementioned problem.

[0006] Claim 1 is directed at a method for achieving the abovementioned object.

[0007] Claim 2 characterizes the basic design of an electric release circuit for achieving the abovementioned object.

[0008] The switching device provided according to the invention ensures that as soon as the safing switch is closed and the controller evaluates the output signal of the accident sensor in such a way that only one firing stage has to be fired in order to protect a person, during a time which is long enough to fire the second stage with such a delay that the person is no longer put in danger, a signal state is maintained which corresponds to the closed safing switch.

[0009] The controller can in this way never independently simulate closing of the safing switch but instead it is only possible to maintain the signal state of a previously closed safing switch during the predetermined time period.

[0010] The subclaims are directed at advantageous embodiments and developments of the release circuit according to the invention.

[0011] The invention can be used advantageously wherever systems, in particular protection systems, are triggered with multiple stages in response to external influences and are protected by means of a safing switch which releases the operation of the protection system.

[0012] The invention is explained below with reference to a block circuit diagram of a motor vehicle occupant protection system which contains the release circuit according to the invention.

[0013] According to the single FIGURE, two firing stages 2 and 4 are connected to outputs of an electronic controller 6. The firing stages 2, 4 are assigned to gas generators (not illustrated) which inflate an airbag. An accident sensor 8 which can contain, for example, one or more acceleration sensors is connected to the controller 6. Reset outputs 10 and 12 are connected to the inputs of a NAND element 14 whose output is connected to the reset input 15 of a logic circuit module 16 which is embodied, for example as a D-flipflop. A signal output 18 of the controller 6 is connected to a signal input 19 of the circuit module 16.

[0014] A safing switch 20 is contained in a bridge circuit 22 which has resistors 24, 26 and 28 and is connected to a voltage source 29. A tap 30 of the bridge circuit 22 is connected to an input of a comparator 32 whose other input is connected to the other tap 34 of the bridge circuit 22.

[0015] A timing element 36 whose output is connected to one input of an OR element 38, whose other input is connected to the output of the comparator 32 in parallel with the timing element 36, is connected to the output of the comparator 32.

[0016] The output of the OR element 38, which, as will be explained below, forms a safing signal output 39, is connected to a control and data input 40 of the circuit module 16 and to the control input of a switch 42 which is embodied, for example, as a transistor. The switch 42 is located between a firing capacitor 44 and the firing stages 2 and 4. An output 46 of the circuit module 16 is connected via a diode 48 to the tap 30 of the bridge circuit 22.

[0017] The design and function of the described component and the basic function of an airbag system are known per se and are therefore not explained.

[0018] The interaction of the circuit elements is described below:

[0019] The safing switch 20 is embodied in such a way that it closes when there is a deceleration above a threshold value, this deceleration being below the deceleration which leads to a triggering of the airbag. The closing of the safing switch 20 leads to the signal level to one input of the comparator 32 changing, as a result of which its output level goes, for example, to a high level and the timing element 36 starts. The expiry time period of the timing element is, for example, approximately 100 ms, this time period being advantageously selected such that it is longer than the longest delay with which the second firing stage, for example the firing stage 4, is fired with respect to the firing stage 2 if both firing stages are fired for optimum protection of a vehicle occupant.

[0020] During the time period during which the timing element 36 is activated, a high level signal is present at the corresponding input of the OR element 38. As long as the safing switch 20 is closed, a high level signal is also present at the other input of the OR element 38. Thus, when the safing switch 20 is closed (the closing period may be very short under certain circumstances) and/or when the timing element 36 is still running, a safing signal which switches the switch 42 to the conducting state is present at the safing signal output 39, so that the firing voltage which is supplied by the firing capacitor 44 is present at corresponding inputs of the firing stages 2 and 4. The firing stages 2 and 4 are thus released for firing, which takes place if the controller 6 generates corresponding firing signals after the output signals of the accident sensor 8 are evaluated. When the safing switch 20 is open and the timing element 36 has expired, the sating signal at the safing signal output 39 ends, and the switch 42 opens, so that the firing stages 2, 4 can normally no longer be fired.

[0021] If the controller 6 evaluates the output signals of the accident sensor 8 to the effect that only the first firing stage 2 has to be fired to provide personal protection, said controller 6 generates, at the signal output 18, a clock signal which is fed to the signal input 19 of the circuit module 16. The safing signal which is present at this time is present at the data input 40 of the circuit module 16. This signal state leads to the situation in which a low level appears at the output 46 of the circuit module 16 and is connected in parallel with the safing switch, which low level places the tap 30 in the same level state as the closed safing switch 20, as a result of which the safing signal is retained at the output of the OR element 38. The switch 42 thus remains closed so that the controller 6 can fire the second firing stage 4 with sufficient delay in order to avoid placing in danger the person who is protected by the firing of only the first firing stage. This firing can take place although the firing switch 20 is open again and the timing element 36 has expired.

[0022] The circuit module 16 is reset by means of a reset pulse which the controller 6 generates with sufficient delay after the firing of the first firing stage at its output 10, or by means of a general system reset pulse which the controller switch generates at its output 12.

[0023] The NAND element 14 feeds these pulses to the reset input of the circuit module 16. The circuit module 16, which is embodied for example as a D-flipflop, thus forms a self-locking circuit which is actuated by signals at its inputs 19 and 40 and maintains its output level until a reset takes place. The diode 48 serves to isolate the bridge circuit 22 from the circuit module 16 in such a way that only the low level at the output 46 and/or the low level which is brought about by the closing of the safing switch 20 are effective.

[0024] The described release circuit which releases the firing capability of the firing stages 2, 4 by means of firing pulses which are generated by means of the controller 6 ensues that the release can be prolonged beyond the period during which the safing switch 20 is closed or the period which is predefined by the timing element 36, only if the safing switch 20 is initially closed and the controller 6 determines that only one of the two firing stages is allowed to be fired in order to protect a vehicle occupant. The circuit component 16 then places its output 46, which is connected in parallel with the safing switch 20, at a level which corresponds to the closed safing switch (for example to ground) so that a signal which corresponds to the closed safing switch 20 is present at the output of the OR element 38.

[0025] The described circuit can be modified in various ways. The firing stages 2 and 4 can be assigned separate firing energy sources which are released by means of separate switches. The comparator 32 is not absolutely necessary; likewise, the NAND gate can be dispensed with if only one reset is provided. The circuit module 16 which forms a logic circuit can be constructed in such a way that it contains a separate timing element which resets it automatically after a predetermined time period. 

1. A method for releasing the firing capability of at least two firing stages (2; 4) which can be fired separately in a vehicle occupant protection system, in which method a safing signal which is used to release the firing capability of the firing stages (2; 4) is prolonged if only one firing stage is fired in order to protect the vehicle occupant when there is an accident.
 2. An electric release circuit for a vehicle occupant protection system having at least two firing stages, containing a safing switch (20), at least one accident sensor (8) and a controller (6) which, as a function of the output signal of the accident sensor, generates control signals for firing only one firing stage (2, 4) or both firing stages (2, 4) simultaneously or with a delay, the safing switch (20) responding to a physical parameter and closing when a threshold value at which it is not yet necessary to provide vehicle occupant protection by means of the protection system is exceeded, and the firing stages (2, 4) being connected in such a way that they can be fired only when a signal which corresponds to the closed safing switch is present at a safing signal output (39) and a firing signal which is assigned to them is present, characterized in that a switching device (14, 16) which is connected to the safing signal output (39) and to the controller (6) is provided which brings about a signal state corresponding to the closed safing switch when the safing switch (20) is closed and when there is a controller (6) and when a controller (6) which brings about the firing of only one firing stage is present at the safing signal output.
 3. The circuit as claimed in claim 2, characterized in that the switching device (14, 16) has a circuit component (16) with a control input (40), a signal input (19) and an output (46), the control input (40) being connected to the safing signal output (39), the signal input (19) being connected to the controller (6), the output (46) being located in parallel with the safing switch (20) and the output (40) assuming a signal stage which corresponds to the closed safing switch after a safing signal which appears at the safing signal output (39) when the safing switch closes is applied to the control input (40) and a clock signal which is generated by the controller (6) when it is necessary to fire only one firing stage is applied to the clock input (19).
 4. The circuit as claimed in claim 3, characterized in that a timing element (36) which is triggered by closing the safing switch (20) is connected downstream of the safing switch (20), in that an output of the timing element is connected to an input of an OR element (38) whose other input is connected to the safing switch (20) and whose output forms the safing signal output (39).
 5. The circuit as claimed in claim 4, characterized in that the safing switching (20) is contained in a branch of a bridge circuit (22), and in that a comparator (32) is provided, the one input of which comparator (32) is connected to the safing switch (32) and to the output (46) of the circuit module (16), and the other input of which comparator (32) is connected to the other branch of the bridge circuit, and the output of which circuit module (16) is connected to the input of the timing element (36) and to the one input of the OR element (38).
 6. The circuit as claimed in one of claims 2 to 5, characterized in that the switching device (14, 16) has a reset input (15).
 7. The circuit as claimed in claim 6, characterized in that the reset input is connected to the output of a NAND element (14) whose inputs are connected to resetting outputs (10, 12) of the controller (6).
 8. The circuit as claimed in claim 3 and 5, characterized in that a diode (48) is provided between the output of the circuit module (16) and the bridge circuit (20).
 9. The circuit as claimed in claim 3, characterized in that the circuit module (16) is formed by a D-flipflop. 